Didehydroaspartate Modification in Methyl-CoenzymeM Reductase Catalyzing Methane Formation

All methanogenic and methanotrophic archaea known to date contain methyl-coenzymeM reductase (MCR) that catalyzes the reversible reduction of methyl-coenzymeM to methane. This enzyme contains the nickel porphinoidF(430) as a prosthetic group and, highly conserved, a thioglycine and four methylated amino acid residues near the active site. We describe herein the presence of a novel post-translationally modified amino acid, didehydroaspartate, adjacent to the thioglycine as revealed by mass spectrometry and high-resolution X-ray crystallography. Upon chemical reduction, the didehydroaspartate residue was converted into aspartate. Didehydroaspartate was found in MCRI andII from Methanothermobacter marburgensis and in MCR of phylogenetically distantly related Methanosarcina barkeri but not in MCRI andII of Methanothermobacter wolfeii, which indicates that didehydroaspartate is dispensable but might have a role in fine-tuning the active site to increase the catalytic efficiency.